Several types of field-effect-transistors (FETs) are available for use at microwave/millimeter-wave frequencies. These high-frequency FETs include metal-semiconductor-field-effect-transistors (MESFETs) and high-electron-mobility-transistors (HEMTs). A HEMT is distinguished from a MESFET in that in a HEMT, charge is transferred from a charge donor layer to an undoped channel layer.
There are generally two types of HEMTs. One type is referred to simply as a HEMT, whereas the other type is referred to as a pseudomorphic-HEMT or PHEMT. The difference between a HEMT and a PHEMT is that in the PHEMT, one or more layers of the PHEMT have a lattice constant that differs significantly from the lattice constant of other materials that compose the device. As a result of this lattice mismatch, the crystal structure of the material forming the PHEMT channel layer is strained. Although this lattice mismatch (and the corresponding strain) makes growth of PHEMTs more difficult than the growth of HEMTs, several performance advantages are obtained. For example, the charge density transferred into the PHEMT channel layer is increased, often resulting in high electron mobility and high electron saturated velocity. As a result, a PHEMT can develop higher power levels and can operate at higher frequencies with improved noise properties as compared to a HEMT.
In particular, the gallium arsenide (GaAs) PHEMT has played a major role in microwave/millimeter-wave amplification and control applications. However, to enable still more capable systems, there continues to be significant efforts aimed at improving the breakdown voltage and the power-handling performance of a GaAs PHEMT. An improvement to the PHEMT structure can be achieved for power and switch devices by emulating the metal-oxide-semiconductor (MOS) device structures commonly found in Silicon-based FET technology. The primary limitation why such a similar device has not found its way into GaAs technology is the notable absence of a viable gate-insulator material.
Previous attempts for such a gate-insulator layer include the use of oxides, sapphire materials, and aluminum oxides. However, these attempts have typically been unsuccessful because once a GaAs wafer is removed from a processing chamber (typically a molecular-beam-epitaxy chamber), the surface of the wafer often forms holes, i.e., “pins”, such that it is difficult to impossible to form an oxide or other type of insulator on the wafer surface in a chemical-vapor-deposition (CVD) chamber.